Phosphate removing granules work! Iron and aluminum phosphates are simply very insoluble. So, these products will remove phosphates from the water. The use of phosphate-removing granules are indicated where a phosphate test indicates high phosphates are causing an algae bloom. “Blue-green algae” blooms are very commonly caused by high phosphate in the water.
So if a 95% water change doesn’t remove the phosphate, phosphate-removing granules are a very good option.
Source: Aquariumscience.org – David Bogert